Pyridine-2,3-dicarboxylate esters are useful intermediates for the preparation of herbicidal 2-(2-imidazolin-2-yl)nicotinic acids, esters and salts, such as those described in U.S. Pat. Nos. 4,562,257; 4,638,068; and 4,647,301.
One procedure for preparing herbicidal 2-(2-imidazolin-2-yl)nicotinic acids from pyridine-2,3-dicarboxylic acid esters is illustrated by the following flow diagram: ##STR1## where R.sup.1 is C.sub.1 -C.sub.4 alkyl, R.sub.2 is C.sub.1 -C.sub.4 alkyl or C.sub.3 -C.sub.6 cycloalkyl, and when R.sub.1 and R.sub.2 are taken together with the carbon to which they are attached they may represent C.sub.3 -C.sub.6 cycloalkyl optionally substituted with methyl; W is O or S; and R.sub.3 and R.sub.4 are hydrogen, halogen, C.sub.1 -C.sub.6 straight or branched alkyl, alkenyl, or phenyl optionally substituted; and R.sub.5 and R.sub.6 are each C.sub.1 -C.sub.4 alkyl.
The preparation of pyridine-2,3-dicarboxylate esters is accomplished in accordance with various synthesis procedures, such as those described in U.S. Pat. Nos. 4,723,011; 4,798,619; and 5,047,542. The described synthesis methods involve the reaction of selected substituted compounds to form a pyridine ring structure having dicarboxylate and other substituents. Illustrative of a synthesis method is the reaction of an .alpha.-halo-.beta.-ketoester with an .alpha.,.beta.-unsaturated aldehyde or ketone in the presence of an ammonium salt. Another synthesis method is by the reaction of 2-aminomaleic acid diester with 2-ethacrolein. A preferred synthesis method is via the reaction of dialkyl N-hydroxyaspartate and a ketone such as 2-ethacrolein as illustrated in Example V of the present specification.
All of the synthetic methods for the production of pyridine-2,3-dicarboxylic acid diesters have in common expensive starting materials, and the subsequent recovery of impure diester product mixtures. Additional product loss is suffered when the diester is hydrolyzed to the corresponding diacid, and the diacid then is recovered and purified before conversion to the desired cyclic anhydride intermediate.
It is noted that Example 1 of U.S. Pat. No. 4,723,011 describes a procedure which is stated to yield 95% diethyl 5-ethylpyridine-2,3-dicarboxylate. All subsequent efforts to duplicate the Example 1 product purity were not successful. In any event, the utility of silica gel column chromatography as a purification procedure is not amenable to commercial-scale operation.
Because of the commercial importance of herbicidal 2-(2-imidazolin-2-yl)nicotinic acid compounds, there is intense interest in new and improved methods of synthesizing pyridine-2,3-dicarboxylic acid diesters, and converting the diesters to the corresponding cyclic anhydrides, with a view to the significant economic consequences.
Accordingly, it is an object of this invention to provide a process for the conversion of a pyridine-2,3-dicarboxylic acid diester to the corresponding cyclic anhydride without isolation and purification of the diacid intermediate.
It is a further object of this invention to provide a process for preparing a pyridine-2,3-dicarboxylic acid diester with a purity of at least about 90 weight percent, and converting the pure diester to the corresponding cyclic anhydride without recovery and purification of the diacid intermediate.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.